1. Field of the Invention
This invention relates to apparatus for forming images to be printed on raster or all-points-addressable printers. It relates particularly to building up the image to be printed from prestored patterns which are stored as pels and more particularly to eliminating or clipping the patterns on a pel basis so that patterns can be overlaid with more precision.
2. Description of Related Art
The pages printed on All Point Addressable (APA) printers are usually stored as raster images, the images being being mapped dot by dot from the raster storage, also referred to as a raster buffer, via the printer to the printing medium. In the simplest case, the pels (dots or picture elements) are black or white, stored in the raster buffer as binary values.
Even at a low resolution such as 240 pels per inch, a standard letter sized page requires a memory capable of storing about 5.4 million bits. The technique most commonly used has been to store in a page memory characters or code words that are keyed to (represent) patterns stored in a pattern store. Typical patterns are letters, punctuation, and the like. Custom characters can be included such as trademarks and business logos, and fonts can be changed or intermixed. According to the commands in the page buffer, patterns are moved from the pattern store to the raster buffer. The raster buffer may contain the pel images for an entire page or a few hundred lines of pels ahead of the pels being printed. Therefore, the page is composed in the page buffer in terms of prestored patterns which are moved to the raster buffer in accordance with the commands from the page buffer.
This approach permits text and graphics to be combined on the same page while permitting pages to be printed at high speeds.
Whether text or graphics, the images can be stored as more than one pattern in the pattern store and composed in the raster buffer by positioning the patterns in adjacent character boxes, i.e., preallocated positions for characters.
Usually, however, it is desirable to be able to vary the space occupied by a pattern by eliminating, trimming, or clipping parts of the pattern.